This invention relates to a thermally conductive compound and method of constructing a low impedance, thermal interface/joint between an electronic component and a heat sink the compound having desired adhesive and closure force characteristics.
Electrical components, such as semiconductors, transistors, etc., optimally operate at a pre-designed temperature which ideally approximates the temperature of the surrounding air. However, the operation of electrical components generates heat which, if not removed, will cause the component to operate at temperatures significantly higher than its normal operating temperature. Such excessive temperatures can adversely affect the optimal operating characteristics of the component and the operation of the associated device.
To avoid such adverse operating characteristics, the heat should be removed, one such method being a conduction of the heat from the operating component to a heat sink. The heat sink can then be cooled by conventional convection and/or radiation techniques. During conduction, the heat must pass from the operating component to the heat sink either by surface contact between the component and the heat sink or by contact of the component and heat sink surfaces with an intermediate medium. In some cases, an electrical insulator must be placed between the component and heat sink. Thus, a heat-conducting path must be established between the component and the heat sink surfaces with or without an electrical insulator therebetween. The lower the thermal impedance of this heat conducting path the greater the conductivity of heat from the component to the heat sink. This impedance depends upon the length of the thermal path between the component and heat sink as well as the degree of effective surface area contact therebetween.
As the surfaces of the heat sink and component are not perfectly flat and/or smooth, a full contact of the facing/mating surfaces is not possible. Air spaces, which are poor thermal conductors, will appear between these irregular mating surfaces and thus increase the path's impedance to conduction. It is thus desirable to remove these spaces by utilizing a heat conducting medium, the medium designed to contact the mating surfaces and fill the resulting air spaces. The removal of these air spaces lowers the path's thermal impedance and increases the path's thermal conductivity. Thus, the conduct of heat along the thermal path is enhanced.
Mica insulators with silicone grease thereon, the silicone grease containing "heat conducting particles," such as a metallic oxide, have been inserted between the component and heat sink to establish a thermal path. The grease can also be applied directly to the mating surfaces in an attempt to fill the resulting voids therebetween. However, the non-soluble grease is messy and can contaminate the equipment, clothing and personnel.
Another proposed solution was to coat a polymeric insulating gasket with a metallic oxide thereon, the gasket being inserted between the component and heat sink during assembly. Such oxides can be expensive, toxic and adhesion to the gasket can be difficult. Moreover, the gasket may not fully mesh with the irregular mating surfaces of the component and heat sink resulting in undesirable, inefficient air spaces therebetween.
The use of a compound comprising a paraffin wax with a softener such as petroleum jelly as the intermediate medium has been proposed in the Whitfield U.S. Pat. Nos. 4,299,715, 4,473,113, 4,466,483. The softener is intended to make the compound less brittle so it will not crack when coated onto the intermediate flexible insulator. However, this compound changes from a solid to a liquid state at the component's normal operating temperature which decreases its thermal conductivity. Also, the compound tends to flow away from the thermal path/joint which increases the impedance of the thermal path. Moreover, this flow can contaminate the surrounding surfaces.
Also, the use of softeners makes the resulting compound more susceptible to abrasion or chemical solvents. Thus, the compound can be rubbed off its substrate carrier during handling or component cleaning. Also, the "blocking temperature" of the compound is lowered, i.e. the temperature at which the coated carriers will stick to each other. (If the blocking temperature is equal to or lower than the room temperature, the coated carriers will stick to each other.) Also, the softeners makes the compound stickier which makes it difficult to manipulate and susceptible to collection of foreign matters thereon. Such foreign materials can lead to component malfunctions, if not failure.
In response thereto I have invented a method of selecting a compound for establishing an efficient thermal joint between the surfaces of an electrical component and heat sink. With cognizance of a normal operating temperature of a selected component, the compound is selected to melt only during initial component operation by either external heat or a component temperature well above the component's normal operating temperature. Once initially liquified or sufficiently deformable, the clamping pressure of the component to the heat sink causes the compound to fill the spaces resulting in the thermal path between the heat sink and the component. This action presents a thermal path of low impedance which initiates an effective conduct of the heat from the component to the heat sink. The component temperature then falls to a temperature below the compound melt temperature and to its normal operating temperature which causes the compound to resolidify. Upon subsequent operation of the component, the component reaches only the component's normal operating temperature as the previously established compound joint formed during initial component operation remains in a solid state. As the compound does not melt during subsequent component operation, a higher thermal conductivity is maintained. Moreover, as compounds of high molecular weight can be used in the above process, a higher thermal conductivity will result with or without the use of heat conductive particles.
As above discussed, the shorter the path between the component and heat sink the lower the thermal resistance. Thus, the lower the force required to reduce the thickness of the thermal compound interface the easier to reduce the thermal resistance of this path. This force must be coordinated with the closure force. By closure force I mean the aforementioned clamping pressure/force needed to initially join the component, thermal interface and heat sink.
It is known to have a film, e.g., a diamond film, along the component interface which serves as a "heat spreader". Any localized heat on the component will be dispersed along the film in all directions (isotropic) which enhances the transmission of the heat from the component to the heat sink.
The diamond films may be rigid, inflexible and fragile. In order to manipulate these films the film must have a thickness of at least a few hundred microns. However, these films result from a slow chemical vapor deposition process. The deposit process is a slow one, i.e., only about one micron/hour. In order to take advantage of the basic thin film and its high thermal conductivity, it is desirable to have a thermal interface compound that can interface the film with the component and heat sink at a very low closure force. Otherwise, the film will break at a high closure force.
It is also desirable that the interface material become flowable during initial component operation and/or deformable under low closure forces but not so flowable as to migrate away from the interface area. However, the interface material should not be so viscous that it requires high closure forces for component mounting which could damage the component or any associated "heat spreader" film.
Thermal resistance and closure forces are thus related. Since thermal resistance is lower when the thermal path is reduced, it is desirable that a very thin interface be formed at low closure forces. Otherwise, a large closure force may damage the component and/or intermediate film.
Most electrical components cannot withstand closure forces of more than 10-20 psi. The diamond film is even more sensitive to closure forces. Known interface materials require hundreds of psi to achieve a path having a low thermal resistance. Thus, a closure force problem exists. It is noted that the ASTM test standard on thermal phase change materials is done at 438 psi, well above the maximum closure force that should be applied to an electronic component. Thus, the problem may not be a recognized one.
It is desirable to have an interface material at room temperature that will change phase to a flowable state at elevated temperatures and/or deformable at low closure forces. The material should not be so viscous that it requires large closure forces to deform so as to obtain the desired component interface. The material should. not migrate away from the component/heat sink under elevated temperatures or under closure forces. A very thin thermal interface at low closure forces should be created to preclude damage to the component and/or heat sink and/or any intermediate film therebetween.
The thermal material need not be used with a substrate carrier. A substrate increases the distance between the electrical component and the heat sink and thus increases the thermal resistance. Thus, the material should be free standing if a carrier is not desired.
Currently, pressure sensitive adhesive (PSA) strips along the edges of the thermal interface material adhere the thermal interface to the heat sink. However, these strips can only partially cover the interface material as the strips have high thermal impedance and increase the thermal path. At times these strips do not provide sufficient adhesion. Moreover, foreign matter can migrate between the PSA strips and the heat sink which increases thermal resistance.
Thus, the thermal interface material should be flexible, easy to handle at room temperature and dry to the touch. It also should flow at a temperature above room temperature and deform under low closure forces. The material should adhere to the heat sink and component surfaces but be removable therefrom by heat application. Also, the interface material should be able to be stored on the heat sink for transport and subsequent use.
In response thereto I have arrived at a process for selecting an interface compound that meets the above objectives as well as presents the following characteristics:
1. The interface material can be manufactured in sheet or roll form, cut to a desired shape and then placed on the heat sink for subsequent: sandwiching between the electrical component and the heat sink or otherwise compressed on the heat sink for adherence upon cooling. PA1 2. The interface material can be melted by either external heat or the heat generated by the initial component operation. PA1 3. Upon cooling below its melt/phase change temperature, the material provides sufficient adhesion to maintain the electric component to the heat sink. Thus, mechanical fasteners, e.g., PSA strips, are not required. PA1 4. As long as the operating temperature of the component remains below the melt/phase change temperature of the thermal interface material, the component remains firmly adhered to the heat sink. PA1 5. The projection of external hot air onto the component will increase the thermal interface temperature so as to reduce the adhesive bond for component removal.
It is therefore a general object of this invention to provide an improved compound and method of selecting the same for reducing the impedance to heat flow through a thermal joint established between an electrical component and a heat sink while providing an effective adhesive bond.
Another object of this invention is to provide a compound and method, as aforesaid, which is initially liquified/deformable during initial component operation but remains in a solid state during subsequent component use.
A further object of this invention is to provide a compound and method, as aforesaid, wherein the compound does not melt at a subsequent normal operating temperature of the component but can be removed upon the application of external heat at a higher temperature thereto.
A more particular object of this invention is to provide a compound, as aforesaid, which is easily coated onto a substrate carrier for placement between the component and heat sink.
Another object of this invention is to provide a compound and method, as aforesaid, which provides a high thermal conductivity relative to previous compounds utilizing material softeners.
A further object of this invention is to provide a compound, as aforesaid, which is easy to manipulate and does not contaminate surrounding personnel and equipment.
Another particular object of this invention is to provide a compound, as aforesaid, which includes a material therein so as to avoid the problems associated with material softeners.
A further object of an embodiment of this invention is to provide a compound which initially adheres the electrical component to the heat sink at a low closure force.
Another object of this invention is to provide a compound, as aforesaid, which deforms under low closure forces of the component to the heat sink.
Still another object of this invention is to provide a compound, as aforesaid, which may be effectively utilized with "heat spreader" type films.
Another particular object of this invention is to provide a compound, as aforesaid, which can be used in sheet, roll or rod form and on printed circuit boards.
Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, an embodiment of this invention.